Due to the explosive growth of broadband communications in homes, network contents are increasingly diversified. Generated along with it, increased communication traffic and expanded communication services are pressing demands for larger capacity, higher speed and greater functionality of the backbone communication networks on a daily basis. In recent years, optical communication technologies have served an important role in meeting these demands. Further, in past optical networks, point-to-point type communication systems connecting between two points by optical to electrical and electrical to optical conversion signal processing have been a mainstream. However, in the future, it is essential to deploy mesh-type communication that connects multiple points using optical signals without conversion to electrical signals on all networks, including access networks, so that more diversified use of communication by each user can be realized.
A waveguide device is an example of components that perform an important role in this optical communication system. In applications of optical interference principle, various functions such as an optical signal splitting/coupling device, a wavelength multiplexing/demultiplexing device, an interleave filter, an optical switch, a variable optical attenuator (VOA), and the like have been achieved. These devices are waveguide-type devices, so they have flexibility in design, and can easily be made on a large scale and with high integration. Not only that, since semiconductor component such as LSI manufacturing processes can be used, it is expected that these devices will be excellent for mass production. While various devices such as waveguides using semiconductors and polymer materials have been put to practical use, especially silica waveguides fabricated on a silicon substrate have properties such as low loss, stability and superior coupling with optical fibers, and are one of waveguide devices for which practical applications have been most advanced.
One of schemes for an optical communication system node constructed using these waveguide devices is a reconfigurable add/drop multiplexing (ROADM) which is used for wavelength division multiplexing (WDM) signal. This scheme has a function of sending all signals to adjacent nodes after transferring with a lower layer network only for arbitrary WDM channel signals in a node, and is mainly used as a scheme for configuring a ring network. The optical devices necessary for achieving this function include a wavelength multiplexing/demultiplexing filter for multiplexing or demultiplexing WDM signal for each wavelength, an optical switch for switching a signal path, a VOA for adjusting optical intensity of a signal, an optical signal transmitter/receiver, an optical intensity monitor and the like, and in particular, a wavelength multiplexing/demultiplexing filter, an optical switch and a VOA and the like can be achieved using waveguide devices.
In recent years, these waveguide devices have been integrated in a single module, making it possible to configure a highly functional optical device which realizes the major functions of a ROADM system, and deployments into actual network systems have actively been put forward. FIG. 14 is, as one example, a block diagram of a circuit integrated with wavelength multiplexing/demultiplexing filters (1404, 1406, 1416), optical switches (1408-1 to N), VOAs (1410-1 to N), optical couplers (1402, 1412-1 to N) and photo detectors (PD) for monitoring (1414-1 to N) as one module 1400. According to the example of FIG. 14, the WDM signal that enters the main path from the input (In) is first split by the tap optical coupler 1402. Then, one of the split signals is separated into individual wavelength signals by a DEMUX filter 1404 for drop path and only the signals having the wavelength used by a lower-layer network is detected. The other signal is also separated into individual wavelength signals by a different DEMUX filter 1406, and then passing through 2×1 optical switches 1408-1 to N which select either the signals from Add path, transmitted from the lower-layer network, or the signals from the main path. In the 2×1 optical switches, the signals from the Add path are selected only for the wavelengths that correspond to the wavelength signals detected earlier in the drop path. Furthermore, the signal level of each wavelength is adjusted by the VOA 1410-1 to N, and the output is monitored by means of the tap optical coupler 1412-1 to N and monitor PD 1414-1 to N connected thereafter, and fed back to control the attenuation at the VOA. The level-adjusted signal of each wavelength become a WDM signal through a wavelength multiplexing (MUX) filter 1416, and then is going out from the output (out) of the main path.
According to a conventional technology, implementation of module is realized by connecting these individual optical devices each other via optical fiber in the module. However, in the future, in order to make a module more compact, larger scale and lower power consumption, further improvement in integration is a substantial challenge.
One of technologies that have been proposed to meet this requirement of improving integration is a multi-chip integration technology. This is a technology in which individual waveguide device substrates are directly connected together without using optical fibers, so that waveguide devices themselves can be smaller and the mounting area inside the module can be reduced. For example, in the configuration of FIG. 14, the wavelength multiplexing and demultiplexing filters 1406, 1416 are manufactured as one waveguide device substrate 1420. Similarly, the optical switches 1408-1 to N, the VOAs 1410-1 to N, and the optical couplers 1412-1 to N are manufactured as one waveguide device substrate 1430. Then, when connecting, the substrates are connected each other directly, not via optical fiber. In addition, the monitor PDs 1414-1 to N are not waveguide devices, but can be connected, not via optical fiber, to the monitor ports for the optical couplers 1412-1 to N, on the end surface of the wavelength multiplexing/demultiplexing filter substrate 1420, or the end surface of the optical switch substrate 1430. With this technique, the length of optical fiber used in the module 1400 and the number of parts for connecting optical fiber to the substrates 1420, 1430 can be eliminated, and as a result, the mounting area inside the module is reduced, improving integration of the devices. In this case, a VOA bears the function of adjusting the optical level of the passing optical signal by an attenuation operation and suppressing level deviation between the channels.
The most basic configuration of a VOA using a waveguide device is illustrated in FIG. 15A. This VOA 1500 is a Mach-Zehnder Interferometer (MZI) type optical device, which comprises two directional couplers 1504 and 1508 for splitting or coupling an optical signal and arm waveguides 1506a, 1505b, with thin film heaters 1512a, 1512b being formed on the arm waveguides 1506a, 1506b. An incident optical signal from a port 1502a is split by the directional coupler 1504, respectively propagating along the arm waveguides 1506a and 1506b, and combined again by the directional coupler 1508. On this occasion, when power is supplied to one of the thin-film heaters 1512a and 1512b from an electrode pad 1516, 1518, a phase difference occurs between the arm waveguides 1506a and 1505b, and the intensity of the optical signal output from the port 1510a or 1510b changes according to the phase relationship in the directional coupler 1508. When the phase difference is 0, the optical signal is output from port 1510b 100%, and when the phase difference is 71, output from port 1510a 100%. Taking advantage of this phenomenon, if this phase difference is adjusted by controlling the power supply to the thin-film heaters in analog fashion, the device can be used as a VOA. FIG. 15B is a cross-sectional diagram at section line XVB-XVB in FIG. 15A. The optical waveguide is fabricated on a silicon substrate 1520 and composed of clad 1522 made of a silica glass and a rectangular shaped core 1524 covered thereby. On both sides of the arm waveguides, there are heat-insulating grooves 1514 formed by removing the clad along the waveguides using an etching technique, which can lower the electric power required for switching or attenuation. Now, based on the MZI interference principle, even when error occurs in the coupling rate due to manufacturing error of the optical couplers, in order to obtain a sufficient extinction ratio or optical attenuation, the path from port 1502a to port 1510b, or the path from port 1502b to port 1510a (cross path) is typically used as the main signal path. Furthermore, when taking into consideration the power consumption or the polarization dependency of thermo-optical effect, most typically the optical signal will be blocked, or driven to achieve the maximum attenuation, when the thin-film heaters 1512a, 1512b are applied with no power. For that, it is required that a predetermined suitable difference (optical path length difference) is given to effective optical distances in the arm waveguides 1506a, 1506b over which the optical signals propagate through the waveguides, that is, the optical path lengths.
The optical attenuation operation in a VOA, having a MZI as the basic element and comprising two optical waveguides, not only attenuates the optical level of the main port (output waveguide connected to optical fiber or to other waveguide device), but also outputs the excess optical power (attenuated power) to other port (dummy port). For example, in a MZI-type VOA that uses a cross path as the main signal path, assuming port 1502a in FIG. 15A is an input, port 1510b becomes the main port and port 1510a becomes the dummy port. According to this conventional art, the excess optical power guided to the dummy port propagates to the output end surface of the waveguide device substrate and is generally emitted to the air directly.
Citation List
Patent Literature
PTL 1: Japanese Patent Publication No, 3755762
Non Patent Literature
NPL 1: Y. Hashizume, et. al., “Compact 32-channel 2×2 optical switch array based on PLC technology for OADM systems”, ECOC2003, MO3-5-4